Piping systems including fired heaters in process plants accumulate deposits which are difficult, time consuming, and expensive to remove. Hydrocarbon-based process fluids are heated in fired heaters to temperatures at which carbon tends to bake onto the pipe walls in extremely hard deposits. These deposits choke down the pipe diameter, requiring extra pumping effort. They also create an undesirable insulation effect in heat transfer equipment and fired heaters. The extra energy required to overcome these resistances to heat transfer and flow increases process costs and can lead to equipment damage and piping rupture. The cleaning process is generally complicated by numerous bends, valves, manifolds, diameter changes, and other variations in the pipe layout.
Fired heaters are often constructed with plugged headers at regular intervals. These plugged headers join parallel sections of straight pipe to create a serpentine flow path as described in U.S. Pat. No. 5,698,042 by Sims et al. which discloses a specially-dimensioned cleaning module, also referred to as a pig, for traversing plugged headers. Within the plugged headers, the cross sectional shape of the pipe typically changes from round to rectangular or oval and back to round.
Any technique for cleaning these piping systems and fired heater systems must have the ability to remove the deposit from varying geometries, maintain its integrity as it travels and conforms to the changes, yet leave the pipe walls undamaged. Any cleaning efficiencies that can be gained in terms of speed, cost, or effectiveness are advantageous.
The practice of using pigs in pipelines to separate or displace products is well established. Likewise there are a wide variety of devices to clean straight runs of pipe. It is only recently that pigs have been developed to clean hard deposits from piping systems containing multiple direction changes, diameter changes, and other variations in pipe geometry. U.S. Pat. No. 5,265,302 by Sivacoe, for example, shows a cylindrical, low-density polymer-based pig with molded-in receptacles having cleaning studs which are screwed into the receptacles. The molding of a Sivacoe pig requires a complicated and time-consuming positioning of numerous receptacles within the mold prior to forming the pig. After the pig body is formed, further time is expended when the pig is assembled by threading each stud into one of the receptacles. The receptacles create voids which weaken the structure of the pig so that the pig has a tendency to separate along lines from receptacle to receptacle. The size and therefore holding power of the receptacles is limited by the voids or discontinuities that they create in the pig body. As the receptacles are made larger and therefore closer together, the tendency for separation increases. When the pig is subjected to the loads of cleaning, these receptacles are sometimes pulled entirely out of the pig body.
Other methods of attaching cleaning studs are described in U.S. Pat. No. 4,244,073 by Sagawa in which a barb is pressed into a foam bodied pig, or U.S. Pat. No. 4,242,771 by Knapp in which a foam body is cast directly around the stud. In both of these cases, the holding power of the cleaning studs is limited by the low resistance of the foam body.
Sivacoe discloses one type of hollow end in U.S. Pat. Nos. 5,265,302 and 5,358,573 which are flared and extend into the pig by only a moderate distance. The primary structure of the Sivacoe pig is a solid cylindrical body. Another pig having a more pronounced hollow end is the Uraflex III pig available from Ura-Flex Manufacturing, Granbury, Tex. The hollow ends are not designed to support or engage a cleaning stud according to the present invention. Instead, the intent of the Sivacoe and Ura-Flex hollow ends is to provide a sealing action for the pig when it is hydraulically driven.
A pig designed to clean fired heaters or complicated piping systems must have sufficient flexibility to negotiate short radius bends, manifolds and headers. It must also have sufficient stiffness to apply a cleaning force against the pipe walls. Existing cleaning pigs are typically cast in a single spherical or cylindrical form. The flexibility of these pigs is typically adjusted by varying the density of the material of construction such as a polyurethane foam. Conventional understanding dictates that a solid cylindrical or spherical pig formed from a solid (non-foam) elastomer such as polyurethane would be too stiff and would not be economic due to the amount of material used. Knapp in U.S. Pat. No. 4,242,771, column 3, lines 27-30 states that "Foam densities in the range of 10-20 pounds per cubic foot are ordinarily acceptable. Higher densities . . . increase cost to an unacceptable level."